In modern wireless receivers of Long Term Evolution (LTE), Worldwide Interoperability for Microwave Access (WiMAX) or Wideband Code Division Multiple Access (WCDMA) telecommunication systems, the received radio signal constellations are processed into soft-bits, each representing a probability of a bit being digital “zero” or digital “one”. The signal constellations are associated with typical modulation schemes such as Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM) 16 or QAM 64. Thereby, each QPSK constellation point produces 2 soft-bits, each QAM16 point constellation point produces 4 soft-bits and each QAM64 constellation point produces 6 soft-bits.
A decoder algorithm such as for instance a Turbo decoder or an Viterbi decoder is exploiting gain from redundancy coding. By using complex processing a decoder algorithm attempts to correctly reconstruct the (encoded) transmitted sequence with highest probability. If a whole data block is decoded correctly and a Cyclic Redundancy Check (CRC) has been passed, an Acknowledgement Information (ACK) is transmitted from the receiver back to the sender in order to inform the sender about a successful data block reception. Otherwise a Not Acknowledgement Information (NACK) is transmitted and, depending on the type of data traffic, the same data block is usually again transmitted from the sender to the receiver. This type of error-control method is called Hybrid Automatic Repeat Request (HARQ).
In particular in LTE, WiMAX and WCDMA telecommunication systems in case of a failed data block reception, retransmission usually follows. However, retransmission may not follow in certain types of data traffic such as for instance Voice over IP (VoIP) data traffic. Normally, if a retransmitted data block is identical to the previous at least partially unsuccessfully transmitted data block, the retransmitted data block would have similar probability for a correct reception as the original one (assuming (a) that fading effects were not affecting the channel quality and (b) that data blocks were being received independently, without being combined together). In order to additionally improve the chance of a successful data signal decoding in retransmission, various schemes may be used.
In standards employing the so called with HARQ “chase combining” method, the same bits are being retransmitted again but the least reliable bits (which were previously transmitted for example as bits 4 and 5 in QAM64) could be retransmitted at the most reliable bit positions 0 and 1. Depending on possibly selected constellation re-arrangements also the “bit position 2” and the “bit position 3” could be used for a reliable bit transfer. For instance in WCDMA QAM16 and QAM64 a proper selected constellation re-arrangement may allow the decoder to collect more reliable information about all bits coded on a specific constellation point.
Another possibility for realizing an effective data block transfer for example in LTE, WCDMA and other telecommunication systems is the so called HARQ “incremental redundancy” method. Thereby, rather than repeating the same bits (although using a different constellation mapping), instead more redundancy or parity bits are being sent. Of course, the number of the additional redundancy or parity bits typically depends on used coding rate and on the used interleaving scheme. A Turbo encoder requires a coding rate of ⅓, which means that three output bits are produced per single input bit.
Known puncturing or interleaving schemes are used in order to select for radio transmission only a subset of the overall encoded bits. Thereby, the effective coding rate may be changed from originally ⅓ to a coding rate of for example ⅔. In retransmissions, different interleaving parameters are selected and different sets of parity bits would be transmitted. Adding more parity bits may mean effectively changing the coding rate from ⅔ of the original transmission down to a coding rate of for example ½ or ⅓ for the retransmission. Typically, interleaving schemes are designed in such a way to avoid the same bits being sent at the same least reliable bit positions.
Due to air interface latencies and various system delays, a typical receiver employing HARQ must be able to handle multiple data blocks: one data block just being received and the other data blocks whose transmission has previously been failed and which are waiting for retransmission. Typically 4 to 6 HARQ processes are deployed. This means that the progress of transmitting 4 to 6 data blocks must be tracked.
When a data block fails a CRC check, its soft-bits are stored in memory and will be combined with new soft-bits being associated with a retransmission. The reason why the old soft-bits are stored is because they also contain valuable information about bit probabilities, which, when being combined with retransmitted soft-bits, will significantly increase the likelihood of successful data block decoding after retransmission. That is why retransmitted data blocks usually have a higher probability for a successful decoding.
In latest 3GPP standards, faster modulation schemes up to QAM64, Multiple Input Multiple Output (MIMO), larger bandwidths and more parallel data codes were added. All of these measures result in an increased data block size and consequently significantly increase the HARQ memory buffer requirements, which significantly increase the overall system memory allocation and thus, the cost for a corresponding radio receiver.
EP 1 337 066 B1 discloses a HARQ retransmission method in a telecommunication system, wherein data packets consisting of identical or partly identical modulation symbols encoded with a forward error correction (FEC) technique prior to transmission are retransmitted based on a repeat request and are subsequently bit-level combined on the basis of soft-information values (soft-bits). The calculation of the soft-bits being input into an FEC decoder comprises the steps of (a) calculating and buffering the soft-information values of the Most Significant Bits (MSBs) of each retransmitted data packet, (b) combining, for matching modulation symbols, the current soft-information values of the MSBs with the buffered soft-information values of at least one of the previous received transmitted packets and (c) calculating the soft information for at least some of the remaining bits from the combined soft-information values of the MSBs. On the receiver side this known HARQ retransmission method reduces the buffer size requirements.